Vacuum pumps typically comprise an impeller in the form of a rotor mounted on a shaft for rotation relative to a surrounding stator. The shaft is supported by a bearing arrangement comprising two bearings located at or intermediate respective ends of the shaft. One or both of these bearings may be in the form of rolling bearings. Usually, the upper bearing is in the form of a magnetic bearing, and the lower bearing is in the form of a rolling bearing.
A typical rolling bearing comprises an inner race fixed relative to the shaft, an outer race, and, located between the races, a plurality of rolling elements for allowing relative rotation of the inner race and the outer race. To prevent mutual contacts between the rolling elements, they are often guided and evenly spaced by a cage. Adequate lubrication is essential to ensure accurate and reliable operations of rolling bearings. The main purpose of the lubricant is to establish a load-carrying film separating the bearing components in rolling and sliding contact in order to minimise friction and wear. Other purposes include the prevention of oxidation or corrosion of the bearing components, the formation of a barrier to contaminants, and the transfer of heat away from the bearing components. The lubricant is generally in the form of either oil or grease (a mixture of oil and a thickening agent).
Vacuum pumps using oil-lubricated bearings require an oil feeding system for feeding oil between the contact areas of the bearing, which enables the oil to perform cooling as well as lubrication and thereby permit the bearings to run at a faster speed. Turbo-molecular pumps have traditionally used a wicking system for supplying oil to a rolling bearing. In such a system, a felt wick partially submerged in an oil reservoir feeds oil to a conical “oil feed” nut mounted on the shaft. With rotation of the pump, oil travels along the conical surface of the nut to the bearing. The oil passes through the bearing and is returned to the reservoir.
In such oil feeding systems, a pressure differential may be generated across the oil reservoir which causes outgassing in the reservoir. In static conditions when the pump is not operating but is being initially evacuated by a primary pump outgassing can cause oil to escape from the bearing cavity and contaminate the pump. Vibrational excitation during use of the pump may additionally promote the nucleation of bubbles contributing to oil loss. Over and above contamination and loss of oil, outgassing may cause an excess of oil to be transferred to the oil feed nut which may stress the bearing.